The present invention is directed to a piezoelectric programmable surface acoustic wave (SAW) filter and a method of use thereof. In particular, the present invention provides a multi-interdigital transducer (IDT) input tunable SAW filter with selectable center frequency and 3 dB bandwidth features.
Surface acoustic wave (SAW) devices are an important class of piezoelectric devices, providing frequency control, frequency selection, and signal processing capabilities. SAW devices have many diverse applications, with the various types of SAW devices including filters, resonators, delay lines, correlators and the like.
SAW filters, in particular, play a key role in telecommunications. They have been widely used as bandpass and spectrum-shaping filters in mobile and wireless applications. Other applications for SAW filters currently available include wide area network(WAN), wireless local area network(WLAiN) communications, cordless phones, pagers and satellite communications. Linear phase SAW filters are used in communications circuits when distortion of the processed signal is to be avoided. SAW filters are preferable to conventional LC filters as they are much smaller, cheaper and more versatile, making them ideal for telecommunication applications.
Coupling between surface waves and electrical signals is usually achieved by interdigital transducers (IDTs). One simple form of the IDT consists of many parallel fingers alternately connected to opposite electrodes to which the signal is applied. The wavelength of the generated elastic wave is equal to the transducer periodicity p, defined as the center distance between two consecutive fingers of one comb of the IDT. Shown in FIG. 1 is the geometry of an interdigital transducer 10. The finger period P equals one wavelength of the surface acoustic wave. It is divided into two fingers and two spacings with the same width of quarter wavelength. The overlaps W of the fingers are all the same. The length L of the IDTs is defined as (Nxc3x97P)/2 where N is the number of overlap.
The basic structure of a SAW filter consists of one input and one output interdigital transducers deposited on a piezoelectric substrate. These IDTs function as a transmitter and receiver for the surface acoustic waves. When an AC voltage is applied to input transducers, due to piezoelectricity the transducer produces mechanical deformation of the piezoelectric substrate surface. The reason for this is the electric field between two opposite transducer fingers of different polarity. The surface acoustic wave travels on the surface until reaching the output IDT where it is transformed back to electrical signal. Shown in FIG. 2 is a schematic diagram of the basic design of a SAW filter 20 with constant finger overlap W and uniform finger spacing. When an AC voltage is applied to the input IDT, it will cause strain, and thus cause the lattice to vibrate. The input IDT converts signal voltage variations into mechanical surface acoustic waves. Vin is the equivalent input voltage and VL is the load voltage. R and RL are the SAW filter""s source resistance and load resistance, respectively. The acoustic wave will propagate along the surface, polarizing the lattice cells, generating an electric field. When this wave reaches the output IDT the electric field will induce a potential difference between neighboring electrodes. The output IDT is employed as a receiver to convert mechanical SAW vibration back into output voltages. For a signal to be transmitted, the change must be continuous. That means only alternating current signals can be transmitted. Therefore SAW filters are always bandpass, or bandstop filters.
SAW filters can be designed to provide quite complex signal processing functions within a single package containing only a piezoelectric substrate with superimposed thin metal film input and output interdigital transducers (IDTs). SAW filters can be mass-produced using semiconductor microfabrication techniques, which enables the outstanding reproducibility of the SAW filters. SAW filters are often implemented in small, rugged, light and power efficient modules, making them more attractive to the other competing technology, such as lumped RLC filters, ceramic block filters, multi-layer ceramic filters and other microwave filters. SAW filters can operate as a bandpass, bandreject, adaptable or matched filter. Such versatility makes SAW filters ideal for mobile and wireless communications.
A SAW IDT, as shown in FIGS. 1 and 2, is a bi-directional device; that is, the acoustic wave will propagate in both directions of the propagation axis. This will cause an inherent 3 dB loss for the power transmitted. At the output, for maximum power transfer, the IDT and load impedance must be matched, resulting in another 3 dB loss. Therefore a SAW filter as presented above has an inherent 6 dB insertion loss. Other loss mechanisms also exist, including mechanical losses due to crystal defects in the acoustic wave medium and diffraction of the wave.
Thus, communication receivers are often faced with the problem of trying to receive a weak signal in the presence of strong interfering signals. Even if the receiver have the sufficient sensibility to receive the weak signal, the receiver will be desensitized to a level determined by the strength of the largest interfering signal and the dynamic range of the receiver. Selective front-end filtering, which this programmable SAW filter provides, is strongly preferred and recommended in such situations.
Current programmable SAW filters are often large, complex and costly compared to regular SAW filters. Moreover, the relatively high insertion loss prevents their usage in many insertion loss sensitive applications. It is therefore desirable to design and fabricate a simple, small, inexpensive and low insertion loss programmable SAW filter for implementation in adaptive communication systems, radar systems and similar applications. Programmability of the SAW filters is highly desirable in such fields so as to optimize the receiver""s sensitivity and allow for adaptive signal processing.
The present invention employs a multi-element input interdigital transducer (IDT) SAW filter having an improved insertion loss level wherein the addition of resistance weighting to the multi-element IDT improved the passband ripple. The present invention can be used in both commercial and military applications such as wideband radar and communication systems employing adaptive filter techniques. It can also be used in spread spectrum communication systems or frequency hopping systems as an adaptive pre-filter to remove interference. It can be integrated into ASIC designs for many communication and wireless applications.
The programmable SAW filter of the present invention is fabricated on 128xc2x0 rotated Y-cut lithium niobate(LiNbO3). The device preferably consists of 11 constant overlap interdigital transducers (IDT) in the propagation path. The IDTs"" center frequencies are designed to be 124.8 MHz, 166.3 MHz and 199.6 MHz, respectively. The transducers are composed of quarter wavelength electrodes with different electrode widths and have the same aperture of 2400 xcexcm. Switchable single-IDT input and multi-IDT input SAW filter configurations are contemplated. The changes between different SAW filter configurations are implemented by computer controlled RF switches.
The programmable SAW filter described herein has both center frequency and bandwidth tunability. The center frequency produced by the programmable SAW filter ranges from 126.8 MHz to 199.1 MHz while the 3 dB bandwidth ranges from 18.8 MHz to 58.9 MHz. The lowest insertion loss of 9.5 dB is achieved. In comparison with the single-IDT input SAW filter configuration, the multi-input filter configuration offers lower insertion loss by taking advantage of the constructive interference of the surface waves. Matching network for the programmable SAW filter has been developed to improve insertion loss level and stopband attenuation. Resistance weighting was implemented and the results showed improved passband response.
It is therefore an advantage of the present invention to provide a programmable SAW filter with switchable input and output IDT. The programmable feature of this device will enable it to be applied as an adaptive filter in the fields of wireless, radar and satellite communications and the like.
It is another advantage of the present invention to provide a programmable SAW filter without complex tap weighting and summing circuits. The programmable SAW filter of the present invention simplifies the device structure and fabrication process by eliminating conventional tap-weighting and summing circuits, significantly simplifying the device structure complexity, and reducing both the size and cost of a programmable SAW device.
It is an additional advantage of the present invention to realize frequency response shaping without apodization.
It is yet another advantage of the present invention to easily realize a multi-IDT input configuration that offers enhanced tunability and filter performance.